AS-Level Chemistry Chapter 5 States of Matter The Liquid State
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The Liquid State
Kinetic theory of liquids
1) In liquids, the particles are packed quite closely together but in a fairly random arrangement(gaps are present between them). So, the particles have limited movements.
2) A liquid has fixed volume but do not have fixed shape.
3) Intermolecular forces are present between the particles, its strength is stronger than in the gaseous state but weaker than in the solid state.
Melting and freezing
1) When a solid is heated, the energy is absorbed by the particles and they vibrate about their fixed positions more vigorously.
2) Then, a point is reached where the particles have energy high enough to overcome the attractive forces that hold them in fixed positions. They break away from their fixed positions and move freely.
3) The solid then becomes a liquid, this process is called melting. The temperature at which this process happens is called the melting point.
4) In freezing, the reverse happens. The liquid particles lose energy until they do not have enough energy to move freely. They are held together in fixed positions again. The liquid solidifies.
Boiling (vapourisation) and condensation
1) When a liquid is heated, the vapour pressure of the liquid increases until eventually it is equal to the atmospheric pressure. Bubbles of vapour will form in the body of liquid.
2) The bubbles then rise to the surface of the liquid, burst open and escape into the atmosphere as a gas. The liquid boils.
3) This happens because when a liquid is heated, the particles absorb energy until it is sufficient to overcome the forces of attraction between them. The particles break away from the fairly close arrangement of the liquid and boils.
4) This process is called boiling. The temperature at which this process happens is called the boiling point.
ii. Conversely, if it boils at a pressure higher than the atmospheric pressure, it will boil slower.
5) In condensation, the reverse happens, the particles lose energy and experience increasing attractive force. They move slower and become closer together when temperature is sufficiently low. The gas liquefies.
Evaporation and vapour pressure
1) The energy distribution of particles in the liquid state follows a shape similar to the normal distribution.
2) The average energy is governed by the temperature. The higher the temperature, the higher the average energy.
4) Unlike boiling, evaporation only takes place on the surface of the liquid.
5) In an open container, the liquid will evaporate until none is left.
6) However, a different thing happens when the liquid is evaporated in a closed container.
7) At first, liquid particles with higher energy escape from the surface of the liquid to become vapour. The vapour particles will collide with the wall of container. The collisions exert a pressure called vapour pressure.
8) As more and more particles escape, the vapour particles become close together. Eventually the particles with lower energy will not be able to overcome the attractive forces between them. The vapour begins to condense and return to liquid.
9) Eventually the vapour particles return to liquid at the same rate as the liquid particles evaporate to form vapour. An equilibrium is reached. At this equilibrium, the concentration of liquid particles and vapour particles remains constant.
10) In this situation, the vapour pressure is maximum and is called the saturated vapour pressure.
11) Vapour pressure will increase when temperature is increased. At higher temperature, more liquid can undergo evaporation. The vapour particles are more energetic and collide with the wall of container harder and more frequently. This causes the pressure exerted to be higher.
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